A large share of energy consumed worldwide is by heat. Cooking, space heating and water heating dominate domestic energy consumption. In the UK, these activities account for 85 percent of domestic energy use, in Europe for 89 percent and in the USA for 61 percent (excluding cooking).

Heat also dominates industrial energy consumption. In the UK, 76 percent of industrial energy consumption is heat. In Europe, this is 67 percent. I could not find figures for the US and for the world as a whole, but these percentages must be similar (and probably even higher on a worldwide scale because many energy-intensive industries have been outsourced to developing countries). Few things can be manufactured without heat. ...

The missing element in our sustainable energy strategy is a renewable source of thermal energy. Geothermal energy produces heat, but its potential is limited to regions that have volcanoes. Biomass is another option, but it faces many problems. If we were to try to provide an important share of heat demand by burning biomass, we would quickly come up against the limits of what the planet can produce. There is only one source of heat energy left, and it is a powerful and inexhaustible one: solar energy.

We tend to see solar energy as yet another way to generate electricity, using photovoltaic panels or solar thermal power plants. But solar energy can also be applied directly, without the intermediate step of generating electricity. Basically, harvesting direct solar energy can happen in two ways: by means of water-based flat plate collectors or evacuated tube collectors, which collect solar radiation from all directions and can reach temperatures of 120 °C (248 °F), and by means of solar concentrator collectors, which track the sun, concentrate its radiation, and can generate much higher temperatures. These can be parabolic trough systems, linear concentrating Fresnel collectors, parabolic dish systems or solar power towers. Almost all of these technologies were developed at the turn of the 20th century.

Solar thermal power versus solar thermal heat

Solar power tower The problem is that we mostly use this technology for the wrong purpose. In today's solar thermal plants, solar energy is converted into steam (via a steam boiler), which is then converted into electricity (via a steam turbine that drives an electric generator).

This process is just as inefficient as converting electricity into heat: two-thirds of energy gets lost when converted from steam to electricity. This is one of the main reasons why the use of solar thermal energy to produce electricity is only cost-effective in deserts. ...

If we were to use solar thermal plants to generate heat instead of converting this heat into electricity, the technology could deliver energy 3 times cheaper than it does today and become cost-effective also in less sunny regions. The crucial difference between solar thermal electricity and other renewables producing electricity is that solar thermal actually starts with heat energy. Thus, contrary to other renewables, the cost of heat energy using the technology is far lower than the cost of electricity, and so it can compete with burning fossil fuels at the thermal level. ...

Without a doubt, solar heat for domestic purposes should continue to be encouraged and a lot of potential remains. But it does not stop there. According to a 2008 report (pdf), which analyses the situation in Europe, the potential for solar heat in industrial processes is even larger than in the domestic market. About 30 percent of industrial heat demand in Europe is below 100 °C (212 °F), which could be delivered by commercially available flat plate collectors (< 80 °C) and evacuated tube collectors (< 120 °C) currently used for domestic purposes.

Another 27 percent of industrial heat demand requires medium temperatures (100 to 400 °C or 212 to 752 °F), which could be reached by improved versions of these collectors (up to 160 °C, see this document) and by commercially available solar concentrator technologies now mostly used for electricity production: parabolic troughs, parabolic dishes and linear concentrating Fresnel collectors.